In recent years, there has been substantial expansion in the number and type of telecommunications equipment in common use in households and offices. For instance, it is not unusual for a subscriber to have multiple pieces of telecommunications equipment coupled to a single telephone subscriber loop, e.g., a single tip and ring wire pair. For instance, a household might have a telephone, a facsimile machine, a computer using a modem, and an answering machine hooked up to a single subscriber loop. Other than for voice communications using a telephone, it is possible for only one piece of equipment to use the loop at any given time. If a second piece of equipment goes off-hook while a first piece of equipment is using the loop to transmit or receive data, the noise and change in loop voltage due to the second piece of equipment going off-hook can cause data errors in connection with the first piece of equipment. Accordingly, many automated telecommunication apparatuses such as fax machines and modems are designed to detect the condition of the telephone line to which they are coupled before they go off-hook. Thus, for instance, if a facsimile machine coupled to a subscriber loop is receiving a facsimile (such that the line is off-hook), a computer modem having this feature will first check if the line is already in an off-hook condition before going off-hook itself and suppress an attempt to go off-hook if it detects that the subscriber loop is already off-hook (i.e, that another piece of equipment on the same line is off-hook).
While there are many ways to determine whether a subscriber loop circuit is off-hook, probably the simplest way is to determine the DC voltage on the line. In the United States, subscriber loops are biased to approximately 48 volts DC (Direct Current) when the line is not in use (i.e, when no equipment coupled to that line is off-hook). If a telecommunication device on the line is off-hook, then the voltage drops typically to somewhere in the range of 20 volts or less.
In the prior art, telecommunication equipment manufacturers have utilized voltage comparator circuits to detect the DC voltage across tip and ring of a subscriber loop and to use the comparator output as an on-hook/off-hook indicator signal. FIG. 1 is an exemplary circuit of the prior art. In the circuit of FIG. 1, a full wave rectifier 12 is coupled across tip 14 and ring 16. The tip′ output 14a from the rectifier is coupled to a voltage divider 17 comprising resistors 18 and 20. The common node 22 of the voltage divider is coupled to one input of a comparator 24. The other input of the comparator is coupled to a reference voltage 26. The output of comparator 24 is coupled through a high voltage barrier circuit, such as optical coupler 28 to a digital signal processor (DSP) 25. A less expensive electrical (as opposed to optical) coupling circuit, whether inductive or capacitive, would not function well in this application. Particularly, the signal across tip and ring can change too slowly to be distinguished from noise by an electrical coupling circuit.
Of course, it will be understood by those of skill in the art that substantial additional circuitry is coupled across tip and ring that is not illustrated in FIG. 1 in order to provide the functionality of the circuit (e.g., to send and receive facsimiles) and that FIG. 1 merely shows the loop status detection circuitry.
The reference voltage and values of the resistors 18 and 20 in the resistor voltage divider 17 are selected so that the comparator output changes state somewhere between the on-hook voltage (approximately 48 volts in the U.S.) and the off-hook voltage (approximately 20 volts in the U.S.). Accordingly, for example, the reference voltage and divider network resistor values can be selected so that the switching point of the comparator is at approximately 30 volts across tip and ring. The digital signal processor is programmed to disable the telecommunication apparatus from going off-hook when the voltage across tip and ring is less than 30 volts. Otherwise, the DSP allows the apparatus to operate normally.
Except for the DSP, the circuit shown in FIG. 1 is specifically dedicated to the aforementioned feature. The DSP typically would be a DSP that already exists in the circuit for performing some or all the functions of the actual device (e.g., facsimile machine) and would merely have additional functionality built into it for receiving the comparator output signal and selectively enabling/disabling the device from going off-hook responsive thereto.
The telecommunication loop detection circuit itself must not disrupt the loop when checking the loop voltage. To do so would, of course, defeat its very purpose. Accordingly, it is an object of the present invention to provide a telecommunication loop condition detector that is simpler and lower in cost than prior art detectors. The telecommunication device must not disturb any call function with any audible noise injection and must not significantly alter the loop impedance.